1. Field of the Invention
The present invention is directed to magnetic recording media and, more specifically, to barium hexaferrite films deposited on a carbon substrate having a silicon nitride intermediate layer (interlayer).
2. Discussion of the Background
Magnetic recording media such as magnetic disks are now widely used for audio, video and computer applications. In a magnetic recording system, recording and reproducing are conducted by means of a magnetic head. A pattern of remanent magnetization is formed along the length of a track or a number of parallel tracks on a magnetic recording media by means of the recording head. The recorded magnetization creates a pattern of magnetic fields which are used to read the data stored in the magnetic recording medium. When the recorded magnetic medium is passed by the same or a similar recording head, the pattern of magnetization can be read by the recording head and the recorded data reconstructed by appropriate electrical processing.
Recent efforts in the field of magnetic recording media have centered on the development of higher areal densities by increasing both the linear recording density as well as the track densities on the magnetic recording medium. Substantial increases in both linear and track densities require improvements in both magnetic recording materials and recording techniques and components.
Magnetic recording requires an interaction between the surface of the magnetic recording medium and the recording head. High density recording requires that the distance between the magnetic recording medium and the recording head be as small as possible. With decreasing distances between the recording head and the surface of the recording medium, however, problems associated with the material properties of the magnetic head and the magnetic recording material arise. The surface of the magnetic recording medium must be finished to a mirror smooth surface to allow the recording head to approach the surface as closely as possible. At these close spacing distances, problems associated with friction and subsequent wear of a magnetic recording layer may result in deterioration of the recording head and medium. The surface smoothness and adhesion of magnetic recording layers to substrates on which they are deposited is, therefore, particularly important with high density recording media.
The standard substrate material for hard disk recording media is high-purity aluminum-magnesium alloy, coated with a nickel-phosphorus (Ni--P) alloy. Glass substrates have also been used but suffer from problems associated with brittleness and fracture during assembly and operation (Magnetic Recording, C. D. Mee and E. D. Daniel, Vol. I, p. 100, 198-203, McGraw-Hill, Inc., 1987). More recently, carbon substrates have been proposed (U.S. Pat. Nos. 5,045,298; 4,716,078).
Magnetic disks having high density memory capacity have been prepared by depositing a magnetic oxide or metallic magnetic films onto the substrate surface. The media film is conventionally deposited by means of a sputtering process (U.S. Pat. No. 4,411,963). Conventional sputtering processes include diode, triode and magnetron sputtering processes (Microchip Fabrication,, P. Van Zant, pp 204-208, Semiconductor Services, San Jose, Calif. 1984).
Magnetic recording materials may be utilized for both longitudinal (horizontal) and perpendicular (vertical) recording. Magnetic thin film thicknesses of 500-5,000 .ANG. are particularly preferred for perpendicular magnetic recording applications. Magnetic thin films having a thickness of about 200-800 .ANG. are particularly preferred for longitudinal recording applications. Many metal alloy thin films have been proposed for these applications. See, for example, U.S. Pat. Nos. 5,063,120; 5,084,152 and 4,654,276. Metal alloy films containing oxygen are also known (U.S. Pat. No. 5,066,552). These films may be deposited by conventional sputtering processes.
The small head-medium distances critical to reaching higher densities on hard disks give rise to disk wear as noted above. Overcoat layers are known to provide a wear-resistant layer and minimize disk wear. Overcoat layers prepared from rhodium, carbon, TiC, TiN, SiC, Cr.sub.2 C.sub.3 and Al.sub.2 O.sub.3 have been suggested (Magnetic Recording. pp. 219-222; U.S. Pat. No. 4,789,598). A protective overcoat layer of hafnia and zirconia has also been proposed (U.S. Pat. No. 5,078,846).
Barium ferrite thin films have been investigated as possible magnetic recording layers for disk recording media. The reasons for this are the excellent hardness and resistance to environmental degradation of the ceramic material, and its high magnetocrystalline anisotropy and square hysteresis loop leading to high recording density. For example, oriented barium ferrite thin films can be formed by epitaxial growth on SiO.sub.2 /Si wafer disks on which a c-axis-oriented ZnO film has been deposited (M. Matsuoka, M. Naoe, Y. Hoshi, J. Appl. Phys., 57(1):4040-4042, 1985). Oriented barium ferrite thin films are obtained using a facing target-type sputtering system and a substrate temperature of 500.degree. C. Barium ferrite films have also been produced on oxidized silicon wafers using a conventional rf diode sputtering system (A. Morisako, M. Matsumoto, N. Naoe, IEEE Transactions on Magnetics, Vol. MAG-22(5):1146-1148, 1986). Crystalline barium ferrite films generally require high substrate temperatures (400.degree.-650.degree. C.) during sputtering (Magnetic Recording, pp. 217).
As indicated above, the sputtering conditions described require a substrate material which is capable of withstanding temperatures of 500.degree. C. in moderate vacuum conditions without degradation of its mechanical properties (smoothness and dimensional tolerance). Present substrate materials, such as aluminum and glass, are not capable of this. For barium ferrite media sputtering, a substrate such as carbon or alumina is necessary. The material used in the experiments described by Hoshi, oxidized silicon, is too delicate to survive in actual disk drive service.
A need exists, therefore, for improved magnetic recording media, in particular disk magnetic recording media, useful for high density magnetic recording.